The Waldspirale ("Forest Spiral") in Darmstadt, Germany, is not your typical apartment complex. For one thing, its designer Friedensreich Hundertwasser abhored straight lines, so the building is U-shaped, with rounded corners inside and out, and every one of its more than 1000 windows are shaped differently.

For another thing, it's got trees growing around it and inside it and on the roof; Hundertwasser called them the "tree tenants". The entire roof is a curving ramp, giving residents access to the forest on top.

And lastly, the outside of the complex is painted in colorful, sinuous stripes that resemble sedimentary layers in rock.